1. Field of the Invention
This invention relates to a hydraulic pressure control apparatus for use in an automatic transmission for an automobile.
2. Description of the Prior Art
In a hydraulic pressure control apparatus for use in an automatic transmission, a speed stage is determined in relation to the throttle position in an intake path and vehicle speed, and usually the shift-down is carried out from a third speed to a first speed through a second speed. Since the speed stage in the second speed and the third speed is performed usually by the engagement of a hydraulic frictional engaging device, the output torque of an engine is decreased when a throttle valve in the intake path is almost fully closed, and the drive torque becomes negative when the speed change is carried out from the third speed to the second one, thereby presenting a problem in that blowing sounds take place in a drive system.
The hydraulic pressure control apparatus proposed to overcome such a problem comprises:
a throttle valve for producing a throttle pressure as a hydraulic pressure related to an intake throttle position; PA1 a governor valve for producing a governor pressure as a hydraulic pressure related to the vehicle speed; PA1 a 1-2 shift valve for changing over the first speed and the second speed in relation to the throttle pressure and the governor pressure; and PA1 line pressure producing means for producing the line pressure related to an intake throttle position; PA1 a throttle valve for producing the throttle pressure as a hydraulic pressure related to the intake throttle position; PA1 a governor valve for producing the governor pressure as a hydraulic pressure related to the vehicle speed; PA1 a 1-2 shift valve for changing over the first speed and the second speed in relation to the throttle pressure and the governor pressure; and PA1 a 2-3 shift valve for changing over the second speed and the third speed in relation to the throttle pressure and the governor pressure so that the line pressure from the 1-2 shift valve in the second speed position acts as a third speed position holding pressure in the direction of reducing the working force caused by the throttle pressure in the third speed position of the D-range and for receiving the hydraulic pressure for the 2-range in the direction opposed to the working force caused by the governor pressure in the 2-range; PA1 compensating pressure producing means for producing the compensating pressure related to the intake throttle position; and PA1 means for exerting the compensating pressure in the 2-range in the direction of counterbalancing the third speed position holding pressure in the 2-3 shift valve. PA1 an input port into which the line pressure is introduced from a manual valve in the 2-range; PA1 an output port for producing the 2-range hydraulic pressure; PA1 a first control port communicating to the input port; PA1 a second control port communicating to the output port; PA1 a drain; PA1 a spring; and PA1 a spool for receiving one directional working force from the hydraulic pressure in the first control port and the spring and the other directional working force from the hydraulic pressure in the second control port respectively to connect the output port to the input port or the drain. PA1 a frictional engaging device for the L-range which is supplied with the first engaging pressure in the L-range to be put into-the engaged condition; PA1 a frictional engaging device for the second speed which is supplied with the second engaging pressure in the second speed to be put into the engaged condition; PA1 a second speed holding valve having an output port for introducing the second engaging pressure to the second speed frictional engaging device and held at a position where the 2-range input port is connected to the output port in the 2-range; and PA1 an L-range shift valve having a first output port connected to the L-range frictional engaging device and a second output port connected to the 2-range input port of the second speed holding valve and introducing in the 2-range the second engaging pressure to the second output port and in the L-range the first engaging pressure to the first output port while connecting the second output port to the drain. PA1 In the 2-range, since the L-range shift valve and the second speed holding valve are in the 2-range position, the second engaging pressure is introduced through the second output port of the L-range shift valve, 2-range input port and output port of the second speed holding valve to the second speed frictional engaging device to put thereby the second speed frictional engaging device into the engaging condition. PA1 a low speed stage frictional engaging device supplied with a hydraulic medium in the low speed stage to be held in the engaging condition; PA1 a high speed stage frictional engaging device supplied with the hydraulic medium in the high speed stage to be held in the engaging condition; and PA1 discharging speed control means for speeding up the discharging speed of the hydraulic medium from the low speed stage frictional engaging device in the up-shift from the low speed stage to the high speed stage when the intake throttle position is almost nil. PA1 an input port communicating to the low speed stage frictional engaging device in the high speed stage; PA1 a first port connected to the drain through an orifice; PA1 a second port as the drain; PA1 a control port communicating to the high speed stage frictional engaging device; and PA1 a spool having a first position for connecting the input port only to the first port and a second position for connecting the input port to the second port, the spool being displaced toward the second position against a spring when the intake throttle position is almost nil.
a 2-3 shift valve for changing over the second speed and the third speed in relation to the throttle pressure and the governor pressure and permitting a line pressure from the 1-2 shift valve in the second speed position to act as the third speed position holding pressure so as to reduce a working force due to the throttle pressure in the third speed position of a D-range.
Accordingly, since in the low throttle position and the 2-3 shift valve the working force due to the throttle pressure is offset by the third speed position holding pressure, the 2-3 shift valve serves to hold the third speed position until the governor pressure reaches a very small value, i.e., the vehicle speed reaches a very small value. On the other hand, the 1-2 shift valve is changed over from the second speed position to the first speed position by the throttle pressure when the governor pressure is reduced and thereby the third speed position holding pressure also disappears in the 2-3 shift valve so that the 2-3 shift valve is changed over from the third speed position to the second speed position. Consequently, the change-over of the 2-3 shift valve from the third speed position to the second speed position is carried out after the second speed position is changed over to the first speed position in the 1-2 shift valve. Therefore, the shift-down from the third speed to the first speed is carried out without shifting to the second speed.
Further in the 2 (second) range, when the vehicle speed drops to a predetermined value, the 2-3 shift valve is changed over from the third speed position to the second speed position and thereafter the 2-3 shift valve has to be held in the second speed position irrespective of the vehicle speed. For that end, the 2-3 shift valve in the 2-range receives the hydraulic pressure for the 2-range in the direction opposed to the working force due to the governor pressure.
On the other hand, since the line pressure is used for the engaging pressure of the frictional engaging device, it is set as a function of the intake throttle position, i.e., the output torque of the engine. However, as above mentioned, when the line pressure is utilized for the third speed position holding pressure in the 2-3 shift valve, this holding pressure varies with the intake throttle position, therefore the vehicle speed in the change-over of the 2-3 shift valve from the third speed position to the second speed position in the 2-range does not reach a predetermined value but unfavorably varies.
Also in the hydraulic pressure control apparatus for use in the automatic transmission for controllably fixing the vehicle speed to the second speed in the 2 (second) range, the engaging pressure needs to be introduced in the frictional engaging device for the second speed irrespective of the position of the 1-2 shift valve in the 2-range Thus, the second speed holding valve is provided in the 2-range which introduces the line pressure as the engaging pressure to the frictional engaging device for the second speed irrespective of operation of the 1-2 shift valve. However, if this second speed holding valve sticks to a position in which the line pressure is introduced to the frictional engaging device for the second speed, the frictional engaging device for L(low) range and the frictional engaging device for the second speed are put into the engaging condition simultaneously in the L-range, thereby causing damage.
Further, when the intake throttle position is almost nil and the up-shift is carried out, if the frictional engaging device for low speed stage and for high speed stage are both of hydraulic pressure system, the release of said device for low speed stage is delayed and the engagement of said device for high speed stage is quickened by the low output torque of the engine. As a result, an overlapping period during which both frictional engaging devices are under the engaged condition may be elongated to produce the negative torque in the frictional engaging device for low speed stage, sometimes causing shocks and uncomfortable blow sounds.